This invention relates to a catalytic process for upgrading a hydrocarbon feed containing alkyl-substituted aromatic compounds, paraffins, and naphthenes using a catalyst formed from crystalline borosilicate AMS-1B and more particularly relates to a method of selectively hydrodealkylating aromatic compounds while converting paraffins and naphthenes whereby a desired product mix is obtained and especially relates to a method to recover pseudocumene from a mixture of close-boiling hydrocarbons.
Several polymethylbenzenes are important intermediate products in the chemical industry. Particular di-, tri-, and tetra-methylbenzenes are oxidized to produce polycarboxylic acids and anhydrides of aromatic acids which are useful in the manufacture of synthetic fibers and plastics. Recent interest in manufacturing anhydrides of the most important aromatic tri-carboxylic acids, trimellitic (1,2,4-benzene-tricarboxylic acid) is due to its use in trimellitic ester plasticizers. The commercial method of producing trimellitic anhydride is by oxidation of pseudocumene (1,2,4-trimethylbenzene).
Pseudocumene, which as a purified raw material constitutes the largest single item of cost in the manufacture of the anhydride, is recovered by super fractionation of existing streams. Such a process, as will be realized, has involved high operation costs and has limited yield.
Fractionation of a mixed aromatics stream such as an extracted, heavy catalytic reformate is suitable to recover purified pseudocumene provided nonaromatics are absent. The close fractionation necessary is done in two fractionators (each containing about 100 trays) which remove, successively, aromatics boiling above and below pseudocumene. Typically, the fractionation feed of C.sub.9 aromatics from reformates contain only about 40 percent pseudocumene. Such low pseudocumene content requires relatively large fractionation towers.
Purity of a pseudocumene feed for oxidation to produce trimellitic anhydride is critical. The presence of other alkyl-substituted aromatic compounds has a detrimental effect on both the yield and purity of trimellitic anhydride. Furthermore, some aromatic impurities undergo oxidation to compounds which inhibit the oxidation of pseudocumene to trimellitic anhydride.
Presence of nonaromatic hydrocarbons and naphthenes in a mixed aromatics stream can make recovery of purified pseudocumene by fractionation economically unattractive or even impossible. As the paraffin concentration in purified pseudocumene approaches 2 wt. % or more, the oxygen consumed in oxidizing the paraffins represents a significant penalty in production costs for a trimellitic anhydride process. For example, a heavy catalytic reformate contains a low concentration of pseudocumene together with significant amounts of paraffins which have normal boiling point temperatures near the normal boiling point temperature of pseudocumene (about 337.degree. F.). Recovery of pseudocumene having a desired purity by fractionation of such a reformate is very difficult or impossible without a pretreatment process.
U.S. Pat. Nos. 4,268,420; 4,269,813; 4,285,919; 4,292,457; 4,292,458; and 4,327,236, all incorporated by reference herein, disclose uses of crystalline borosilicate AMS-1B in a catalyst for various hydrocarbon conversion processes. These processes include hydrocracking, hydrodealkylation, disproportionation of aromatics, isomerization of normal paraffins and naphthenes, and especially the isomerization of alkylaromatics, such as xylenes. Crystalline borosilicate AMS-1B formulated as a catalyst including an alumina matrix material and a catalytically-active metal such as molybdenum simultaneously can isomerize xylenes and predominantly convert ethylbenzene through hydrodeethylation.
The process of this invention provides a method to convert alkyl-substituted aromatic compounds by selective hydrodealkylation of ethyl, propyl, and butyl groups. Because of the difficulty of removing some paraffins and naphthenes from pseudocumene by distillation due to closeness of boiling points and nonideal behavior of such mixtures, it is desirable to convert paraffins and naphthenes to other hydrocarbon species by hydrocracking or isomerization. A process which permits a single catalyst composition simultaneously to convert paraffins and naphthenes and selectively hydrodealkylate aromatic compounds would be useful. A process which permits both without having any significant effect on the pseudocumene in the feed would be very advantageous.